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Monitoring hamstring and quadriceps strength using handheld dynamometry in patients after ACL reconstruction: A prospective longitudinal study
⁎Corresponding author: Wouter Welling. w.welling1989@gmail.com
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Received: ,
Accepted: ,
This article was originally published by Reed Elsevier India Pvt. Ltd. and was migrated to Scientific Scholar after the change of Publisher.
Abstract
Abstract
Strength data can help in guiding the return to sports (RTS) process in patients after an anterior cruciate ligament reconstruction (ACLR). There is need for a fast, cheap, and portable method to monitor muscle strength in patients after ACLR in an ambulatory stetting. Therefore, the purpose of the current study was to assess the use of a hand-held dynamometer (HHD) to monitor hamstring and quadriceps strength after ACLR and to assess the changes over time in patients tested at 3, 6 and 9 months after ACLR. It was hypothesized that patients show greater muscle strength at 6 months after ACLR compared to 3 months, and at 9 months after ACLR compared to 6 months. Forty-four amateur athletes (n = 44) after ACLR participated. Isometric hamstring and quadriceps strength were measured with a HHD at three time points during rehabilitation. Absolute isometric hamstring strength significantly progressed over time. Similar, absolute isometric quadriceps strength significantly progressed over time. This study showed that using handheld dynamometry can be a simple method to monitor the development of absolute isometric hamstring and quadriceps strength in patients after ACLR.
III, prospective case series.
Keywords
Anterior cruciate ligament reconstruction
Return to sports
Testing
Muscle strength
Handheld dynamometry
1 Introduction
In pivoting sports like football, handball, and basketball, athletes have a relatively high risk of a rupture of the anterior cruciate ligament (ACL).1 After a reconstruction of the ACL (ACLR), more than 90 % of patients expect to return to sports (RTS) without any restrictions.2 The reality is that only 55 % of patients return to competition sports and for those who RTS one in five will sustain a second ACL injury, unfortunately.3,4
The most frequent question a patient will ask after the ACLR is: when can I RTS5?However, the decision when patients can RTS is multifactorial and therefore one of the most complex and challenging decisions to make for members of the treatment team.6 Before RTS, it is therefore recommended to collect physical patient data which can help in guiding the RTS process.7 Therefore, RTS test batteries are developed. The last decade, many studies emphasized the importance of sufficient knee flexion and extension strength as one of multiple important criteria for the progression of rehabilitation phases and requirements for RTS.7–10 Therefore, lower limb muscle strength testing is often an important part of RTS test batteries. The gold standard for measuring knee flexion strength (hamstring strength) and knee extension strength (quadriceps strength) is an isokinetic dynamometer.11 However, there are some disadvantages since this method of testing is relatively expensive, time-consuming, non-portable and therefore, not available for every clinician in daily practice.12–15
Handheld dynamometry is an alternative for isokinetic strength testing.11 Handheld dynamometer strength testing is relatively easy to use, relatively fast, portable, and cheap and is therefore practical to implement by clinicians in daily work.11–15 Furthermore, testing hamstring and quadriceps strength with a handheld dynamometer (HHD) has shown to be reliable and valid since it showed moderate to good validity with the isokinetic dynamometer.11,15
However, to the best of the author's knowledge, no studies are conducted in which isometric hamstring and quadriceps are measured over time using handheld dynamometry to monitor the development of muscle strength in patients after ACLR in an ambulatory setting. Therefore, the purpose of the current study was to assess the use of a HHD to monitor hamstring and quadriceps strength after ACLR. The aims were to (1) assess the changes in hamstring and quadriceps strength over time in patients tested at 3, 6 and 9 months after ACLR, and (2) to compare the results between patients after ACLR and healthy controls. It was hypothesized that patients show greater hamstring and quadriceps strength at 6 months after ACLR compared to 3 months, and at 9 months after ACLR compared to 6 months. Furthermore, it was hypothesized that the healthy controls show greater hamstring and quadriceps muscle strength compared to the patients on all three timepoints.
2 Materials and methods
A prospective study was conducted at PRO-F physical therapy practice, Enschede, The Netherlands by the DUNE (Dutch North East) sports injury study group. The institutional review board (IRB) of PRO-F physical therapy approved this study (IRB nr: 2023001). Data collected for regular care purposes were anonymized and used for the present study unless patients had opted-out for use of their medical data for research purposes.
2.1 Participants
2.1.1 Patients
Patients (1) 18–35 years of age, (2) that underwent a primary ACLR, (3) participated in pivoting sports, (4) had the ambition to RTS, and (5) undergoing rehabilitation at PRO-F physical therapy practice were eligible to be included in the study. Patients were excluded if (1) they experienced pain during testing or (2) the presence of swelling of the injured knee. Patients underwent ACLR at the OCON Centre for Orthopaedic Surgery and Sports Medicine by one of the two orthopedic surgeon authors (A.v.H. or R.H.). ACLR was performed with a hamstring tendon graft (HT) or bone-patellar tendon-bone graft (BPTB) based on the preference of the patient, combined with anterolateral ligament reconstruction with a modified Lemaire technique. All patients received the same, structured, criteria-based rehabilitation program, which was a combination the Melbourne protocol and a progressive strength training program.16,17
2.1.2 Controls
The inclusion criteria for the control group were as follows: 1) age between 18 and 35 years old, and 2) no history of ACL or other ligamentous knee injuries. The control group was tested once, before the start of the 2023/2024 season.
2.2 Procedure
All patients were recruited at the same physical therapy center at PRO-F physical therapy and tested by the same researcher (W.W.), who was not blinded. Before testing, participants completed a warm-up of 10-min stationary cycling or 10-min walking on a treadmill at 5 km/h. Patients were tested at three time points during rehabilitation: at 3 months after ACLR, 6 months after ACLR, and 9 months after ACLR. Absolute isometric hamstring and quadriceps strength was collected using a HHD (microFET; Hoggan Scientific, LLC; West Jordan, UT). During muscle strength data collection, the patients were belt-stabilized which means that patients needed to produce maximal strength on a belt in stabilized positions. This belt-stabilized method has shown to be valid (r > 0.86) and reliable (ICC = 0.88).18 The hamstring was always tested first, followed by the quadriceps. For the patients, the non-injured leg was always tested first followed by the injured leg. For the control group, the dominant leg was tested first followed by the non-dominant leg. The dominant leg was defined as the preferred leg to kick a ball.17 All participants practiced three times with submaximal isometric contractions with each leg, followed by three maximal isometric contractions with 30 s of rest between each contraction. The highest value (Newton) was used for data analysis.
For isometric hamstring testing, the patient was lying in prone position on the table with the arms next to the body. The tested leg was flexed 30° in the knee. The HHD was placed 2 cm proximal of the malleoli of the ankle, on the Achilles tendon. The tester used a belt to keep the HHD on the same position while testing. During the test, the participant was instructed to flex the knee and push as hard as possible for 3 s19 (Fig. 1).
For isometric quadriceps testing, the patient was sitting at the end of the table, with the arms crossed. The tested leg was flexed 60° in the knee. The HHD was placed just proximal from the ankle, on the tibia. The tester used a belt to keep the HHD on the same position while testing. During the test, the participant was instructed to extend the knee and push as hard as possible for 3 s19 (Fig. 2).
2.3 Baseline characteristics
Patient baseline and intraoperative characteristics included in the study were sex, age, injured side (ACLR group) or dominant (control group) side, body mass index (BMI), presence and treatment of concomitant cartilage and meniscal injuries, and time from ACLR to measurement of hamstring and quadriceps strength with the HHD.
2.4 Data reduction
All collected data was saved in a Microsoft Excel database (Microsoft, version 16.79.1.). Limb symmetry index (LSI) values were calculated for peak hamstring and quadriceps muscle strength by dividing the absolute strength of the injured leg with the absolute strength of the non-injured leg x 100.20 For the control group, LSI values were calculated for peak hamstring and quadriceps muscle strength by dividing the absolute strength of the non-dominant leg by the absolute strength of the dominant leg x 100, for both hamstring and quadriceps strength.17 Data was analyzed for the complete groups, as well as for males and females separately. For the between group analysis, the comparison between the injured leg of the ACLR group and the dominant leg of the control group was made since the injured leg was the dominant leg for most of the patients (65.9 %, n = 29).
2.5 Outcome measures
The primary outcome measures were the development of absolute isometric hamstring and quadriceps strength, LSI values, hamstring/quadriceps (H/Q) ratio, hamstring strength normalized to body weight (H/BW), and quadriceps strength normalized to body weight (Q/BW) measured over time (at 3, 6, and 9 months postoperatively) in patients after ACLR. The secondary outcome measure(s) were the comparisons of absolute isometric hamstring and quadriceps strength, LSI values, H/Q ratio, H/BW and Q/BW between ACLR patients and healthy controls.
2.6 Statistical analysis
With an effect size of 0.50 (medium effect ANOVA) and an alpha of 0.05, 34 patients after ACLR were required to obtain a power of 0.80 based on isometric quadriceps and hamstring muscle strength as outcome measures (21; G∗Power, Version 3.1.7)).
Descriptive results are presented as mean ± standard deviation. All data were normally distributed as analyzed with Statistical Packages for Social Sciences, version 20 (IBM SPSS 244 Inc, Chicago, IL). To determine differences in absolute isometric hamstring and quadriceps strength, LSI values, hamstring/quadriceps (H/Q) ratio, H/BW, and Q/BW, across time (3 months, 6 months, and 9 months), between legs (non-injured leg and the injured leg) and groups (ACLR group and control group), a 6 × 3 × 2 ANOVA were conducted. In addition, a 2 × 2 ANOVA was conducted to determine differences in absolute strength and LSI values between patients with a HT ACLR and patients with a BPTB ACLR. Statistical significance was set at p < 0.005 level of confidence.
3 Results
During the study period from January 2022 till August 2023 11 of 58 patients that underwent rehab after ACLR at PRO-F physical therapy did not meet the inclusion criteria and we had missing data of 3 patients due to several reasons (Fig. 3). In total, 44 patients were included.
3.1 Baseline characteristics
Baseline characteristics are presented in Table 1.
| ACLR | CTRL | |||||
| Subgroup | Total | Males | Females | Total | Males | Females |
| N | 44 | 28 | 16 | 58 | 30 | 28 |
| Age | 28.8 ± 9.6 | 29.6 ± 8.6 | 27.3 ± 11.5 | 23.5 ± 4.8 | 23.9 ± 5.6 | 23.1 ± 3.9 |
| Length (cm) | 177.3 ± 9.7 | 183.1 ± 11.2 | 172.9 ± 9.2 | 176.4 ± 8.6 | 182.0 ± 10.1 | 171.2 ± 8.9 |
| Weight (kg) | 76.8 ± 12.3 | 82.8 ± 10.0 | 66.2 ± 7.9 | 69.4 ± 8.5 | 74.6 ± 8.1 | 63.8 ± 4.4 |
| BMI | 21.5 ± 3.4 | 23.2 ± 2.8 | 18.5 ± 2.2 | 19.4 ± 2.4 | 20.9 ± 2.3 | 17.8 ± 1.2 |
| Graft Type (n) | HT (31), BPTB(13) | HT (19), BPTB(9) | HT (12), BPTB(4) | N.A. | N.A. | N.A. |
| Testing timepoint | 3.2 ± 0.4; 6.4 ± 0.7; 9.3 ± 0.6 | N.A. | N.A. | N.A. | N.A. | N.A. |
| Type sport (n) | Football (29), Field hockey (5), Handball (5), Basketball (3), Volleyball (2) | Football (20), Handball (5), Basketball (3) | Football (9), Field hockey (5), Volleyball (2) | Football (58) | Football (30) | Football (28) |
| Tegner Activity Level | 7.1 ± 1.3 | 7.3 ± 0.9 | 6.9 ± 1.5 | 10.0 ± 0.0 | 10.0 ± 0.0 | 10.0 ± 0.0 |
3.2 Primary outcome measures
The results of the development of all strength parameters over time for ACLR patients are presented in Table 2 and Figs. 4–7. Absolute hamstring and quadriceps muscle strength, H/BW and Q/BW for the injured leg both progressed significantly over time for the complete ACLR group, as well as for males and females separately. The only exception was for absolute isometric quadriceps strength and Q/BW for the injured leg between 6 and 9 months after ACLR for females, which did not significantly differ between 6 and 9 months after ACLR (Table 2). Additionally, the LSI values for both isometric hamstring and quadriceps muscle strength significantly increased over time (LSI hamstring 3 months vs. 6 months p-value<0.001; 6 months vs. 9 months p-value<0.001; LSI quadriceps 3 months vs. 6 months p-value<0.001), except for the LSI quadriceps strength, which did not significantly differ between 6 and 9 months after ACLR. The H/Q ratio was significantly lower in the injured leg at 3 months and 6 months compared to the non-injured. At 9 months after ALCR, no significant differences were found in the H/Q ratio between legs.
| Total | Group | Leg | Timepoint | Mean ± SD (N) | P-value: between legs/progression | LSI | H/Q ratio | P-value: between legs/progression | Normalized strength (N/kg) | P-value: between legs/progression |
| Isometric hamstring strength | ACLR | Injured | 3 | 183.9 ± 69.7 | <0.001∗/N.A. | 74.7 ± 17.8 | 0.46 ± 0.12 | 0.027∗/N.A. | 2.4 ± 0.9 | <0.001∗/N.A. |
| ACLR | Non-injured | 3 | 243.9 ± 59.3 | N.A./N.A. | 0.50 ± 0.09 | N.A./N.A. | 3.2 ± 0.7 | N.A./N.A. | ||
| ACLR | Injured | 6 | 227.8 ± 61.4 | <0.001∗/<0.001∗ | 86.9 ± 13.4 | 0.46 ± 0.08 | <0.001∗/0.425 | 3.0 ± 0.7 | <0.001∗/<0.001∗ | |
| ACLR | Non-injured | 6 | 260.9 ± 53.7 | N.A./0.001∗ | 0.50 ± 0.06 | N.A./0.408 | 3.4 ± 0.6 | N.A./<0.001∗ | ||
| ACLR | Injured | 9 | 250.0 ± 55.6 | <0.001∗/0.001∗ | 93.9 ± 9.4 | 0.49 ± 0.07 | 0.077/0.010∗ | 3.3 ± 0.7 | <0.001∗/<0.001∗ | |
| ACLR | Non-injured | 9 | 266.7 ± 54.8 | N.A./0.057 | 0.50 ± 0.08 | N.A./0.222 | 3.5 ± 0.6 | N.A./0.020∗ | ||
| CTRL | Dominant | N.A. | 277.4 ± 57.0 | 0.078/N.A. | 102.6 ± 12.5 | 0.58 ± 0.11 | 0.024∗/N.A. | 4.0 ± 0.8 | 0.079/N.A. | |
| CTRL | Non-dominant | N.A. | 271.9 ± 54.3 | N.A./N.A. | 0.56 ± 0.10 | N.A./N.A. | 3.9 ± 0.7 | N.A./N.A. | ||
| Isometric quadriceps strength | ACLR | Injured | 3 | 395.6 ± 92.1 | <0.001∗/N.A. | 80.2 ± 7.8 | – | – | 5.2 ± 1.0 | <0.001∗/N.A. |
| ACLR | Non-injured | 3 | 496.2 ± 120.3 | N.A./N.A. | – | – | 6.4 ± 1.2 | N.A./N.A. | ||
| ACLR | Injured | 6 | 495.7 ± 107.5 | <0.001∗/<0.001∗ | 93.6 ± 8.0 | – | – | 6.5 ± 1.0 | <0.001∗/<0.001∗ | |
| ACLR | Non-injured | 6 | 532.3 ± 121.1 | N.A./<0.001∗ | – | – | 6.9 ± 1.0 | N.A./<0.001∗ | ||
| ACLR | Injured | 9 | 513.4 ± 115.8 | 0.019∗/0.015∗ | 97.4 ± 8.5 | – | – | 7.1 ± 1.0 | 0.004∗/0.006∗ | |
| ACLR | Non-injured | 9 | 527.1 ± 108.3 | N.A./0.106 | – | – | 6.8 ± 1.1 | N.A./0.041∗ | ||
| CTRL | Dominant | N.A. | 486.7 ± 102.6 | 0.124/N.A. | 98.6 ± 8.7 | – | – | 7.0 ± 1.3 | 0.102/N.A. | |
| CTRL | Non-dominant | N.A. | 493.6 ± 96.7 | N.A./N.A. | – | – | 7.1 ± 1.2 | N.A./N.A. | ||
| Males | Group | Leg | Timepoint | Mean ± SD (N) | P-value between legs/progression | LSI | H/Q ratio | P-value: between legs/progression | Normalized strength (N/kg) | P-value: between legs/progression |
| Isometric hamstring strength | ACLR | Injured | 3 | 204.5 ± 68.5 | <0.001∗/N.A. | 75.8 ± 19.7 | 0.47 ± 0.14 | 0.181/N.A. | 2.5 ± 0.8 | <0.001∗/N.A. |
| ACLR | Non-injured | 3 | 269.2 ± 48.4 | N.A./N.A. | 0.50 ± 0.07 | N.A./N.A. | 3.3 ± 0.6 | N.A./N.A. | ||
| ACLR | Injured | 6 | 255.0 ± 45.9 | <0.001∗/<0.001∗ | 88.8 ± 13.1 | 0.47 ± 0.06 | 0.031∗/0.420 | 3.1 ± 0.6 | <0.001∗/<0.001∗ | |
| ACLR | Non-injured | 6 | 287.8 ± 35.9 | N.A./0.007∗ | 0.49 ± 0.06 | N.A./0.456 | 3.5 ± 0.5 | N.A./0.004∗ | ||
| ACLR | Injured | 9 | 279.2 ± 38.2 | 0.036∗/<0.001∗ | 95.7 ± 9.8 | 0.49 ± 0.06 | 0.265/0.031∗ | 3.4 ± 0.6 | 0.004∗/0.001∗ | |
| ACLR | Non-injured | 9 | 293.4 ± 38.4 | N.A./0.244 | 0.50 ± 0.07 | N.A./0.275 | 3.6 ± 0.5 | N.A./0.117 | ||
| CTRL | Dominant | N.A. | 309.4 ± 51.0 | 0.193/N.A. | 103.3 ± 11.4 | 0.57 ± 0.12 | 0.044∗/N.A. | 4.2 ± 0.9 | 0.090/N.A. | |
| CTRL | Non-dominant | N.A. | 301.8 ± 50.3 | N.A./N.A. | 0.54 ± 0.10 | N.A./N.A. | 4.1 ± 0.8 | N.A./N.A. | ||
| Isometric quadriceps strength | ACLR | Injured | 3 | 431.9 ± 71.3 | <0.001∗/N.A. | 79.4 ± 8.1 | – | – | 5.2 ± 0.8 | <0.001∗/N.A. |
| ACLR | Non-injured | 3 | 546.6 ± 93.5 | N.A./N.A. | – | – | 6.6 ± 0.8 | N.A./N.A. | ||
| ACLR | Injured | 6 | 547.1 ± 92.9 | <0.001∗/<0.001∗ | 93.0 ± 7.1 | – | – | 6.6 ± 1.0 | <0.001∗/<0.001∗ | |
| ACLR | Non-injured | 6 | 589.6 ± 101.7 | N.A./<0.001∗ | – | – | 7.1 ± 1.0 | N.A./<0.001∗ | ||
| ACLR | Injured | 9 | 567.1 ± 104.6 | 0.052/0.040∗ | 97.2 ± 6.7 | – | – | 7.0 ± 1.0 | 0.004∗/0.012∗ | |
| ACLR | Non-injured | 9 | 582.2 ± 90.2 | N.A./0.294 | – | – | 7.3 ± 1.0 | N.A./0.031∗ | ||
| CTRL | Dominant | N.A. | 553.2 ± 80.6 | 0.304/N.A. | 98.2 ± 10.0 | – | – | 7.5 ± 1.2 | 0.119/N.A. | |
| CTRL | Non-dominant | N.A. | 563.9 ± 68.1 | N.A./N.A. | – | – | 7.6 ± 1.2 | N.A./N.A. | ||
| Females | Group | Leg | Timepoint | Mean ± SD (N) | P-value between legs | LSI | H/Q ratio | P-value: between legs/progression | Normalized strength (N/kg) | P-value: between legs/progression |
| Isometric hamstring strength | ACLR | Injured | 3 | 147.8 ± 57.4 | <0.001∗/N.A. | 72.7 ± 14.3 | 0.44 ± 0.11 | 0.017∗/N.A. | 2.3 ± 0.9 | <0.001∗/N.A. |
| ACLR | Non-injured | 3 | 200.0 ± 50.9 | N.A./N.A. | 0.50 ± 0.11 | N.A./N.A. | 3.0 ± 0.8 | N.A./N.A. | ||
| ACLR | Injured | 6 | 180.1 ± 56.5 | <0.001∗/<0.001∗ | 83.5 ± 13.7 | 0.44 ± 0.11 | 0.004∗/0.490 | 2.8 ± 1.0 | <0.001∗/<0.001∗ | |
| ACLR | Non-injured | 6 | 213.7 ± 47.1 | N.A./0.022∗ | 0.50 ± 0.08 | N.A./0.407 | 3.3 ± 0.8 | N.A./0.012∗ | ||
| ACLR | Injured | 9 | 201.0 ± 44.8 | 0.001∗/0.031∗ | 91.0 ± 8.3 | 0.48 ± 0.10 | 0.091/0.083 | 3.1 ± 0.8 | <0.001∗/0.013∗ | |
| ACLR | Non-injured | 9 | 222.1 ± 49.2 | N.A./0.100 | 0.51 ± 0.09 | /0.323 | 3.4 ± 0.8 | N.A./0.041∗ | ||
| CTRL | Dominant | N.A. | 243.2 ± 41.5 | 0.562/N.A. | 101.9 ± 13.9 | 0.59 ± 0.10 | 0.268/N.A. | 3.8 ± 0.7 | 0.268/N.A. | |
| CTRL | Non-dominant | N.A. | 239.9 ± 38.1 | N.A./N.A. | 0.58 ± 0.10 | N.A./N.A. | 3.8 ± 0.6 | N.A./N.A. | ||
| Isometric quadriceps strength | ACLR | Injured | 3 | 332.1 ± 91.7 | <0.001∗/N.A. | 81.6 ± 7.1 | – | – | 5.0 ± 1.2 | <0.001∗/N.A. |
| ACLR | Non-injured | 3 | 408.0 ± 112.7 | N.A./N.A. | – | – | 6.2 ± 1.6 | N.A./N.A. | ||
| ACLR | Injured | 6 | 405.7 ± 63.2 | 0.035∗/<0.001∗ | 94.7 ± 9.4 | – | – | 6.2 ± 0.9 | 0.020∗/<0.001∗ | |
| ACLR | Non-injured | 6 | 432.0 ± 80.9 | N.A./0.201 | – | – | 6.5 ± 0.9 | N.A./<0.109 | ||
| ACLR | Injured | 9 | 423.9 ± 69.8 | 0.368/0.216 | 97.9 ± 11.1 | – | – | 6.5 ± 1.1 | 0.116/0.105 | |
| ACLR | Non-injured | 9 | 435.1 ± 65.0 | N.A./0.214 | – | – | 6.8 ± 1.0 | N.A./0.095 | ||
| CTRL | Dominant | N.A. | 415.5 ± 71.3 | 0.618/N.A. | 99.1 ± 7.2 | – | – | 6.5 ± 1.2 | 0.319/N.A. | |
| CTRL | Non-dominant | N.A. | 418.3 ± 58.2 | N.A./N.A. | – | – | 6.6 ± 1.0 | N.A./N.A. | ||
Comparing the graft types, greater absolute isometric hamstring strength was found at the injured leg for patients with BPTB compared to HT (193.8 ± 48.8 for BPTB vs. 144.4 ± 62.5; p = 0.049) at 3 months after ACLR. Furthermore, hamstring LSI values for patients with BPTB were greater at both 6 and 9 months after ACLR compared to patients with HT (6 months 93.8 ± 10.0 for BPTB vs. 80.1 ± 13.5 for HT; p = 0.013, 9 months 99.8 ± 6.6 for BPTB vs. 90.6 ± 8.1 for HT; p = 0.016). Also, patients with HT ACLR had a greater LSI value for quadriceps strength at 9 months postoperatively compared to patients with BPTB ACLR (98.8 ± 8.7 for HT vs. 90.4 ± 9.1 for BPTB, p = 0.0011).
3.3 Secondary outcome measures
Fifty-eight (n = 58; 30 males and 28 females) age and gender matched professional football players of multiple primary division clubs served as a control group. For males, the control group showed greater absolute isometric hamstring strength, greater hamstring LSI values, a higher H/Q ratio and greater H/BW in the dominant leg compared to the injured leg of the ACLR group at all three timepoints (Fig. 4). At 6 and 9 months after ACLR, the patients showed similar absolute isometric quadriceps strength in the injured leg compared to the dominant leg of the control group (Fig. 4), but lower Q/BW at 6 months. In addition, at 9 months the Q/BW was similar between groups.
Similar to males, for females, greater absolute isometric hamstring strength, greater hamstring LSI values, a higher H/Q ratio and greater H/BW were found in the dominant leg of the control group compared to the injured leg of the ACLR group at all three time points (Fig. 6). At 6 and 9 months after ACLR, patients showed similar absolute isometric quadriceps strength and Q/BW in the injured leg compared to the dominant leg of the control group (Fig. 6).
4 Discussion
The most important finding of the present study is that both absolute isometric hamstring and quadriceps muscle strength significantly increased over time in patients after ACLR, when measured with a HHD. Furthermore, both male and female patients showed similar absolute isometric quadriceps strength and Q/BW compared to age and gender matched professional football players at 6 and 9 months after ACLR. However, the control groups showed higher H/Q ratios and H/BW at these time points. Finally, the present study showed that handheld dynamometry is a relatively easy method to monitor the progression of isometric hamstring and quadriceps muscle strength in an ambulatory setting during rehabilitation in patients after ACLR.
To our knowledge, this is the first study in which lower leg isometric strength measured with a HHD is monitored at three different points during rehabilitation in patients after ACLR. Repeated measurements over time are important to monitor patients' progression and to adjust individual rehabilitation programs. For example, some patients have more difficulty with regaining hamstring strength, and other with regaining quadriceps strength. This requires clinical reasoning by the (sport)physiotherapist from the test results to the content of the rehabilitation program. Monitoring muscle strength is important to individualize the rehabilitation program instead of the traditional “one-size-fits-all” principle.22 Another advantage of repeated measurements is that patients get insight in the effects of the individual training program which might potentially increase the patient's motivation.
For the injured leg, the progression between 3 and 6 months and between 6 and 9 months after ACLR for both absolute isometric hamstring and quadriceps strength is higher than the minimal detectable change (MDC) between 14.1 and 15.1 N.23 This indicates a clinically relevant improvement in both absolute isometric hamstring and quadriceps strength over time. Compared to isokinetic dynamometer testing, testing with a HHD generates less information. However, research emphasizes that isometric strength testing with a HHD is a reliable and valid method for lower leg muscle strength testing12,15 and therefore, is a good ambulatory alternative to measure strength development in patients after ACLR. In addition, fixing the HHD during measurements by using the belt-stabilized positions in the test protocol in the current study increase the reliability and validity in measuring isometric quadriceps strength.12 Handheld dynamometer strength testing is relatively easy to use and therefore practical to implement by clinicians in daily work. Furthermore, it is less expensive compared to isokinetic testing. The test protocol in the current study can be used as an example of how and when patients after ACLR can be monitored over time.
The LSI values of both hamstring and quadriceps strength of patients after ACLR progressed over time. However, lower LSI values for hamstring strength were found for the ACLR patients at all three timepoints compared to the control group. This might indicate a lack of hamstring strength for the ACL patients. This is further supported by the lower H/Q ratios and lower H/BW at all timepoints for both males and female patients compared to the age and gender matched controls. A lower H/Q ratio in combination with decreased hamstring strength is suggested to be a risk factor for an ACL injury, especially in female patients.24 More attention is therefore needed in regaining hamstring strength during ACL rehabilitation to optimize H/Q ratios.
Patients showed similar LSI values for quadriceps strength at 6 and 9 months after ACLR compared to the healthy controls. This might indicate that patients have sufficient quadriceps strength based on LSI values. However, as many studies emphasize caution is needed when using only LSI values since this method can mask bilateral deficits and therefore overestimate performance.11 It is advised to use a combination of absolute values and LSI values when analyzing strength data. Of caution, RTS after ACLR involves more than only regaining sufficient muscle strength. For example, other essential aspects like jump-landing performance, movement technique, plyometrics, change of direction movements, agility and psychological readiness for RTS are important,25–27 but were not investigated in this study. Additionally, research suggested the possible neuroplastic changes in the central nervous system since an ACL injury should be seen as a mild neurological insult.28 This indicates that brain dynamics should be a key rehabilitation target29 and emphasizes the complexity and multifactoriality of RTS after ACLR.
The results of the absolute isometric quadriceps strength of patients after ACLR are higher compared to other published data. For example, Almeida et al. presented lower absolute isometric quadriceps data compared to the absolute isometric quadriceps data presented in the current study.11 One of the reasons of these differences might be related to the differences in test moments in time after surgery between the current study and others. We tested our patients at three timepoints during rehabilitation (at 3, 6, and 9 months after ACLR) which is different compared to testing after 24 months after ACLR.11 Our patients were still within their rehabilitation program in which they followed the progressive strength training program every week. This is different compared to patients who already finished their rehabilitation and therefore might do less strength training. Also, earlier studies found that the progressive strength training program used in the current study results in greater quadriceps strength compared to the traditional standardized program.17 This is in line with the results of the current study and might explain the differences in muscle strength data with earlier studies.
In this study, the control group were professional football players and the ACLR patients were recreational athletes (Tegner level 10.0 ± 0.0 for the control group vs. 7.1 ± 1.3 of the ACL patients). This difference influences the results of the study significantly and might explain the differences in absolute isometric hamstring strength, LSI values and H/BW between groups. Ideally, we included level and sport matched controls however, we do not have this data for this study. Therefore, more normative data is needed from different levels of healthy athletes for isometric muscle strength. On the other hand, both males and females showed no differences in absolute isometric quadriceps strength and Q/BW at 9 months after ACLR when compared to the control group. This might be a surprising result since the control group were professional football players and the ACLR patients were not. On the other hand, this indicates that the rehabilitation protocol, including progressive strength training, used in the current study was effective. This is in line with previous research.17 We therefore advise to include a rehabilitation program with progressive load to prevent underloading to prepare our patients for RTS.
The current study showed differences in absolute isometric muscle strength between graft types within the ACLR group. For example, greater hamstring strength in the injured leg was found in patients after ACLR with a BPTB graft compared to patients after ACLR with a HT graft. This indicates that graft type significantly influences the development of regaining muscle strength during the rehabilitation. This is in line with previous research showing different outcomes regarding muscle strength test results for different graft types.30 Rehabilitation protocols should be tailored based on the surgical procedure.
The results of the current study at 9 months after ACLR might indicate that patients have sufficient quadriceps strength for RTS, and therefore low risk for second ACL injury. However, there is conflicting evidence in the relation between passing RTS criteria and potential risk for second ACL injury.31–36 For example, a systematic review found no differences in passing RTS criteria between patients who sustain a second ACL injury and patients who did not.34 Therefore, we believe that a paradigm shift is needed in the use of current muscle strength tests and criteria for RTS. Instead of traditional strict criteria, lower leg muscle criteria should be used as minimal requirements for on-field rehabilitation. As Buckthrope et al. proposed, RTS is not a static moment in time but rather a dynamic continuum including four different phases: on-field rehabilitation, return to training, return to competition, and return to performance.25 Passing strength criteria should be seen as a minimal requirement for entering the first phase of the RTS continuum with the start of the on-field rehabilitation program.
4.1 Limitations
The current study is not without limitations. First, patients were tested in a closed, clinical, non-ecological environment without fatigue circumstances.37 There is a large difference between clinical testing during rehabilitation and on-field performance during pivoting sports that require patients to react to opponents, teammates, ball, and different surfaces. It can therefore be questioned if this method is valid for RTS testing. Secondly, it was not investigated if patients successfully RTS. Importantly, the control group included professional football players while the ACLR patient group were amateur athletes from various sports. Furthermore, the control group includes equal numbers of males and females whereas the ACLR group includes more males than females. This makes it harder to compare test results between groups. More normative data is needed from different levels of athletes for isometric muscle strength to better evaluate rehabilitation progression.
5 Conclusion
Patients after ACLR progressed in isometric lower leg muscle strength over time. This study showed that using handheld dynamometry can be an accessible and practical method to monitor the development of absolute isometric hamstring and quadriceps strength in patients after ACLR in an ambulatory setting. At 9 months after ACLR, no differences in isometric quadriceps strength were found between patients and healthy controls.
CRediT authorship contribution statement
Wouter Welling: Conceptualization, Methodology, Formal analysis, Investigation, Resources, Data curation, Writing – original draft. Jan Paalman: Conceptualization, Methodology, Resources, Writing – review & editing. Ralph Speerstra: Conceptualization, Methodology, Resources, Writing – review & editing. Albert Van Houten: Conceptualization, Methodology, Resources, Writing – review & editing. Roy Hoogeslag: Conceptualization, Methodology, Resources, Writing – review & editing.
Ethical approval
This study was approved by the institutional review board (IRB) of PRO-F physical therapy approved this study.
Funding statement
The authors did not receive any financial support for the research, authorship, or publication of this article.
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